Caption: Martian impact craters. Computer artwork of a view from space across impact craters and the Argyre impact basin (bright area) in Mars' Southern Highlands. About 4 billion years ago an asteroid or comet collided with Mars creating this surface feature. A thousand miles across, Argyre is the second-largest impact basin on Mars after Hellas Planitia. The impacting object that created the basin is believed to have been about 30 miles in diameter. Within the basin are the relatively flat plains of Argyre Planitia.